This invention relates generally to power amplifier circuits, and more particularly to a system and method for providing input-signal-dependent power-amplifier output-voltage rails.
Typically, amplifiers must be able to amplify a wide range of input signals. This usually requires the amplifier to have large power-supply xe2x80x9crailsxe2x80x9d (high-voltage supplies that connect across the output transistors that are used to xe2x80x9cdrivexe2x80x9d the load). A problem arises, however, when an amplifier is amplifying small signals. In amplifying small signals, there is a small voltage that is developed across the load and, also, a current, which passes through the load as well as through the transistors. Unfortunately, the power transistors drop the balance of the supply voltage across themselves while supplying the load current. This leads to a substantial dissipation in power across the power transistors, and, hence, a substantial inefficiency with respect to amplification of small signals when using large voltage rails.
Efforts have been made to mitigate this problem by using additional voltage supplies (i.e., voltage sources), each having a different voltage potential, thereby allowing the voltage rails to switch from one voltage source to another as the magnitude of the input voltage changes. This approach allows for greater efficiency because the power dissipation may be minimized by tracking the voltage rails to the magnitude of the input signal. However, this approach is cumbersome because it requires multiple voltage sources in order to realize the different voltage rails. In light of this deficiency, there is a need in the art for a system and method of increasing power-amplifier efficiency without increasing the number of voltage supplies.
The present invention provides a system and method for selectively providing varying voltages to an amplifier circuit as a function of the input voltage.
Briefly described, a system and method are provided for variably adjusting voltage rails, whereby a passive voltage supply circuit is charged by a primary voltage supply circuit, and the charged passive voltage supply circuit is used to provide additional voltage to a power amplifier when an input signal exceeds a predetermined threshold voltage.
In architecture, the system comprises a capacitor circuit that is connected to several switches. These switches, when configured in charge mode, connect the capacitor circuit between two voltage potentials, thereby allowing the capacitor to charge. Additionally, the switches, when configured in active mode, place the capacitor circuit serially between the voltage source and the power amplifier, thereby supplying added voltage to the amplifier. The system further comprises a control circuit, which controls the switching function described above. The control circuit comprises a comparator circuit, which compares an input signal to a predetermined threshold voltage. When the input voltage remains below the predetermined threshold voltage, the control circuit places the switches in charge mode, thereby charging the capacitor circuit. However, when the input voltage exceeds the predetermined threshold voltage, the control circuit places the switches in active mode, thereby using the stored charge to supplement the voltage source and, hence, increasing the voltage rail.
In accordance with another aspect of the present invention, a method is provided for selectively varying the voltage rail of a power amplifier as a function of the input voltage. The method can be broadly conceptualized as charging a capacitor circuit, comparing an input signal with a predetermined threshold voltage, and producing a control signal that selectively activates the capacitor circuit to provide added voltage to the amplifier when the input signal exceeds the predetermined threshold.
Other systems, methods, features, and advantages of the invention will be, or become, apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within the scope of the invention, and be protected by the accompanying claims.